This invention relates to a colour pickup tube for use in a single tube type or a two tube type colour television camera system, and more particularly to a colour pickup tube provided with a transparent film formed on the rear surface of an optical colour analyzing stripe filter.
In a colour pickup tube of the type referred to above, a frequency separating system or a phase separating system is generally utilized, but for the purpose of description, a colour pickup tube for a single tube type colour television camera system utilizing the frequency separating system will be considered herein. A colour pickup tube of this type comprises a transparent substrate on the forward end of the tube which confronts an object and an optical colour analyzing stripe filter deposited on the rear surface of the glass substrate for reflecting red and blue colour components contained in the incident light, as disclosed in the specification of U.S. Pat. No. 3,291,901. Within the envelope of the colour pickup tube are disposed a transparent electroconductor film, a photoconductor film and an electron gun to the rear of the optical colour analyzing stripe filter in the order mentioned. In a colour pickup tube of this construction, the red colour component of the incident light from an object is amplitude modulated by an optical colour analyzing stripe filter for reflecting red light whereas the blue colour component is amplitude modulated by an optical colour analyzing stripe filter for reflecting blue colour. These amplitude modulated signals are passed through bandpass filters and detectors to obtain blue and red signals. Since the brightness component is not modulated by the optical colour analyzing stripe filters it is possible to obtain the brightness signal by merely passing the outputs of the filters through a low pass filter.
To manufacture such a colour pickup tube it is difficult to form a transparent electroconductor film and a photoconductor film directly on the optical colour analyzing stripe filters for reflecting red and blue colours, which are deposited on the transparent glass substrate. More particularly, a Nesa (trade mark) film is usually used as the transparent electroconductor film but since the Nesa film comprises a film comprising essentially of tin oxide which is formed by spraying finely divided particles of a solution consisting essentially of stannic chloride on the transparent glass substrate preheated to about 500.degree.C and then decomposing and oxidizing the stannic chloride, it is inevitable to heat the glass substrate to a temperature of about 500.degree.C. For this reason, it is almost impossible to form the optical colour analyzing stripe filters for reflecting red and blue colours by utilizing organic materials of poor heat resistant property and evolving a large quantity of gas in vacuum. Where the optical colour analyzing stripe filters for reflecting red and blue colours are made of interference filters, irregularities are formed on the photoconductor film deposited on the interference filters corresponding to the surface irregularities thereof, thus rendering nonuniform the electric field of the photoconductor film. As a result, the signal current will become abnormal due to abnormal surface electric field which is caused by said surface irregularities. Further, impurities deposited on or contained in the irregularities or the delicate construction of the interference filters impairs the photoelectric converting characteristic of the photoconductor film whereby false image signals corresponding to said surface irregularities will be produced.
To solve this difficulty, it has been proposed to interpose between the colour analyzing stripe filter and the transparent electroconductor film a transparent film with a flat surface on the side thereof facing the transparent electroconductor film. Such a transparent film may be a thin glass film or an overcoating formed by vapour deposition or sputtering of transparent substance, and the thickness of the transparent film should be uniform and may range from 5 to 50 microns which is necessary not to make obscure the image of the colour analyzing stripe filter projected on the photoconductor film.
However, where a transparent film is formed on the transparent glass substrate on the fore end of the colour pickup tube, while the assembly is cleaned, the chemical of the detergent tends to permeate into the interface between the transparent film and the optical colour analyzing stripe filter. Such chemical or moisture permeated into the interface causes the transparent film to peel off. The thinner is the film, the larger is such tendency. Moreover, the light flux arriving at the optical colour analyzing stripe filter via an objective lens will have a large focusing angle at the central portion of the stripe filter whereas a small focusing angle on the peripheral portion thereof. For this reason, the image of the optical colour analyzing stripe filter projected on the photoconductor film will be severely obscured at the central portion, but not so severely on the peripheral portion.